This application relates to an optical demultiplexing system and method such as may be employed in the system and method described in U.S. provisional application relating to a high performance optical multiplexer and demultiplexer being filed on the same day as the present application by David Boertjes and Kim Roberts, to be assigned to Nortel Networks, under the Nortel Networks reference of 13587RO.
The invention relates to an optical demultiplexing system and method, and particularly to a system and method for splitting an optical signal carrying a number of information channels at different frequencies.
Optical communications systems increasingly use wavelength division multiplexing (WDM) in which a number of distinct optical signals are transmitted at different wavelengths, generally down an optical fiber. For example, optical communication in the so-called xe2x80x9cCxe2x80x9d band may allow the transmission of 40 channels, or frequencies, at regular intervals, each carring 10 Gb/s of data. One optical signal can be transmitted at each frequency down a single optical fiber. Other bands and/or other numbers of channels may be used, for example, 56 channels in the xe2x80x9cLxe2x80x9d band, each carrying 10 Gb/s.
A key component in WDM systems is a demultiplexer for splitting apart optical signals at a plurality of wavelengths into the individual channels at individual wavelengths. This may be done using a splitter and a number of different filters tuned to the individual frequencies, by components that demultiplex the light directly, or a combination of these approaches.
One approach to filtering and demultiplexing is to use an arrayed waveguide grating (AWG), also known as a phased-array device. The operation and design of AWGs is described, for example, in xe2x80x9cPHASAR-Based WDM-Devices: Principles, Design and Applicationsxe2x80x9d, Meint K. Smit, IEEE Journal of Selected Topics in Quantum Electronics, Vol. 2, No. 2, June 1996.
FIGS. 1 to 3 illustrate an AWG device. FIG. 1 shows a top view of the AWG, FIG. 2 a side section through a waveguide of the AWG and FIG. 3 is a detailed top view of part of the AWG device. A plurality of optical waveguides 3 are devil on a substrate 1 in a known way. For example, to define the waveguides a buffer layer 5 may be deposited on the substrate, a core 7 deposited along part of the buffer layer to define the waveguide 3 and a cladding layer 9 provided to cover the core and buffer layers. The refractive indices of the buffer 5, core 7 and cladding 9 layers are selected so that light is guided along the waveguide in the region of the core.
The arrayed waveguide device includes an array 11 of waveguides 3 arranged side by side on the substrate and extending between an input star coupler 13 and an output star coupler 15. The input and output star couplers 13,15 are defined by a wide core region in which light can travel freely in the two-dimensional plane of the substrate. This region is known as the free propagation region. Input 17 and output 19 optical waveguides are provided to feed input light into the array 11 of waveguides and to output light from the array respectively. There may be a plurality of input waveguides 17 or output waveguides 19.
As an example FIG. 3 illustrates the output star coupler of a system with a singe input waveguide and a plurality of output waveguides. The ends 21 of the array of waveguides 11 are usually on a geometric circle 23 of radius r whose centre is at the centre 25 of an image plane 27. The output waveguides 19 are arranged on the image plane, which also constitutes a circle. Note that the centres of the circles are not coincident, and may not have equal radii.
The length of the individual waveguides 3 of the array 11 differ (see FIG. 1) and the shapes of the star couplers 13, 15 are chosen so that light input to the input optical waveguide 17 passes through the array 11 of waveguides and creates a diffraction pattern on the output waveguide or waveguides, such that light of a predetermined cent wavelength creates a central interference peak at the centre 25 of the image plane. Light with frequencies slightly higher or lower than the predetermined central frequency is imaged with a central interference peak slightly above or below the centre of the image plane.
In order to achieve this result the optical path length difference between adjacent waveguides of the array is chosen so that it is an integral multiple of the central wavelength. Accordingly, light at the central wavelength which enters the array of waveguides in phase will also leave in phase and thus will create the central diffraction spot at the centre of the image plane. Light with a slightly different frequency will arrive at the output star coupler with slight phase differences across the array, which will cause the light to be imaged to a spot on the image plane a little away from the central spot.
Accordingly, the plurality of output waveguides arranged on the output plane receive light of slightly different frequencies. Equally spaced output waveguides correspond to equally spaced frequencies, at least to a first order of approximation.
FIG. 3 shows the effect of one or more output waveguides connected to the output star coupler 15. It is alternatively or additionally possible to arrange a plurality of input waveguides on the input star coupler with the some effect.
An AWG has a number of properties. One important property is that the distance of the image spot along the image plane as a function of wavelength is substantially linear in wavelength, for wavelengths around the central wavelength. Accordingly, it is possible to separate a number of signals with regular channel separations by positioning output waveguides at substantially regular intervals along the output plane.
A second important property is that the AWG has a repeat frequency. In other words, the interference properties as a function of frequency repeat with a period in the frequency domain. This period is known as the free spectral range (FSR). The free spectral range is a function of the difference in optical length between adjacent waveguides; a large optical length difference results in a small FSR and vice versa.
Although an AWG can carry out demultiplexing, it is not generally practical to demultiplex a large number of channels using a single AWG. For example, it can be impracticable to demultiplex each of the 40 channels in the C-band using a single AWG, for four reasons. Firstly, the C-band covers some 4 000 GHz so the AWG would need an FSR of at least this much. This would result in small path length differences between each waveguide of the array of the AWG and hence a physically large AWG device. Secondly, it would be necessary to provide 40 output waveguides, which would also lead to a large device. Thirdly, the accumulated cross-talk into one channel from the other thirty-nine channels may be excessive. Finally, in some applications it is desired to process a group of channels, e.g. for dispersion compensation, so a multi-stage process might be preferred.
However, all alternative system with separate AWGs in each frequency range would greatly increase the parts count of an optical system and would likewise be inconvenient and difficult to manufacture.
Accordingly, there is a need for an improved optical demultiplexer capable of accurately dividing an optical signal having a moderate or large number of optical channels into individual channels.
Furthermore, in some cases there is a need to apply some processing on optical signals in broad frequency bands as well as to divide the optical signal into marrow frequency bands or individual channels.
Further, the manufacturing costs of optical components can be considerable and it would be beneficial to reduce these costs.
In a first aspect of the invention there is provided an optical system, comprising; an optical splitter for splitting an input optical signal between optical outputs; and a plurality of demultiplexers of like design, each demultiplexer having a plurality of input guides and a plurality of output guides, each input guide having a predetermined frequency range for which optical signals input into the input glide are divided between the plurality of output guides according to frequency; wherein the optical outputs of the optical splitter are connected to respective demultiplexers through input guide having a plurality of different predetermined frequency ranges.
By providing a plurality of inputs for the demultiplexers it is possible for each demultiplexer to demultiplex a subset of the frequency band of the signal input to the splitter, without having to cope with the whole band. This avoids any difficulties in providing a demultiplexer capable of demultiplexing a whole band. For examples in a system demultiplexing 40 channels in the C band, it is very difficult to design a single demultiplexer capable of dividing the 40 channels between 40 outputs evenly and with a suitably low insertion loss.
Moreover, by using a plurality of demultiplexers of like design, the arrangement becomes simple to manufacture. It is only necessary to connect a number of like components to the outputs of the splitter; there is no need to stock a variety of different parts or to fine tune the demultiplexers once in place. All that is required is to connect the correct input.
The demultiplexers may be arrayed waveguide gratings with a plurality of input waveguides. Arrayed waveguide gratings with large numbers of outputs tend to have different losses depending on the location of the output waveguide. These differences can be minimised by having a reduced number of output waveguides and using a number of input waveguides.
The predetermined frequency ranges of the input guides may span a predetermined input light frequency range without overlapping, so that the input light frequency range can be split into narrow frequency ranges, which may correspond to individual channels.
The optical splitter may include filtering functionality, for example at least one coarse filter, for dividing light into predetermined broad frequency ranges output on respective optical outputs which in turn are connected to input guides having corresponding predetermined frequency ranges. In this way, cross-talk between channels can be reduced. Any light that is output on the wrong output of the optical splitter, because of imperfections in the coarse filter, will be passed to a demultiplexer optical input that does not correspond to its frequency, and so the demultiplexer will not pass the light.
According to a second aspect of the invention there is provided an optical system for optical processing of an optical signal divided into a plurality of predetermined groups of channels, comprising: a plurality of optical outputs, each optical output outputting signals in a respective frequency range that corresponds to a predetermined group of channels; a plurality of demultiplexers of like design, each demultiplexer having a plurality of input guides and a plurality of output guides, each input guide having a predetermined frequency range for which optical signals input into the input guide are divided between the plurality of output guides according to frequency; wherein the optical outputs are connected to respective demultiplexers, each optical output connected to the respective demultiplexer through an input guide having a predetermined frequency range corresponding to the frequency of the respective frequency range of the optical output.
The use of demultiplexers of like design greatly simplifies the manufacture of the device. There is no need to stock a number of different types correspond to different frequency ranges. By like design is meant designs that are largely identical or equivalent. The demultiplexers may be of substantially identical design.
The demultiplexers may have cyclic properties that repeat in frequency with a period of the free spectral range. In particular, the demultiplexers may be arrayed waveguide gratings.
A coarse frequency splitter can be used for dividing an input optical signal into the predetermined groups of channels.
Separate dispersion compensation may be provided for each of the predetermined groups of channels. In this way, the dispersion compensation can be separately optimised for each frequency range.
It is also possible to provide separate gain control for the different channels or groups of channels.
Switches may be provided on the inputs of the demultiplexers for switching the outputs of the optical processing section into the corresponding inputs. In his way the switches can be configured in a separate operation to manufacture, which simplifies manufacture.
The demultiplexers may demultiplex signals in frequency bands corresponding to each input and reject signals in adjacent frequency bands. In other words, signals in a frequency band adjacent to the frequency band corresponding to an input are not distributed into any of the outputs of the demultiplexers. This can reduce cross-talk.
The demultiplexers may be AWGs.
The AWGs may be arranged to demultiplex the signals of broad frequency range to which they are attached by realising that the AWGs have properties that repeat in frequency with a period of the free spectral range. Thus, for each input a different group of frequencies of predetermined width within each free spectral range is demultiplexed and at least one other group of frequencies of predetermined width is rejected. The inputs of the AWGs connected to the outputs of the coarse filter can then be selected to be cyclic with increasing frequency of the respective broad frequency range wherein the cycle repeats with a repeat frequency substantially equal to the free spectral range such that each AWG demultiplexes the signal corresponding to the broad frequency range of the output of the coarse filter to which it is attached.
For simplicity, the AWG may have two configurations and the AWGs connected to the outputs of the coarse filter in ascending frequency order may be configured alternately in each of the two configurations. However, this is not essential and the AWG may have a larger number of configurations. For example, if the AWG has three configurations the cyclical configurations in ascending frequency order may be first, second, third, first, second, third, first . . . etc.
The provision of a number of inputs giving effectively a number of configurations of the filters of common design brings advantages over the provision of only a single configuration. Consider first a comparative example of an AWG with a single configuration as the demultiplexer. Each broad frequency range might include 4 channels spaced 100 GHz apart, and an AWG of 400 GHz free spectral range and four outputs might be provided for each broad frequency range output to split the broad frequency range into individual channels.
This arrangement would have the disadvantage that it would mean that the coarse filter had to be very precise with sharp edge cutoffs. In commercial devices, if the first broad frequency range corresponded to channels 1 to 4 it is likely that some signal in channel 5 would also be transmitted into the output for channels 1 to 4 since real coarse filters will transmit some signal a little outside the passband. If only a single configuration of AWG were used, then any signal in channel 5 which was transmitted into the AWG on the output of the coarse filter corresponding to channels 1 to 4 would be output from the channel 1 optical output of the AWG.
For example, in embodiment of the present invention there may be two inputs on the like AWGs, one input for demultiplexing channels 1 to 4, channels 9 to 12 etc, and the other for channels 5 to 8, channels 13 to 16, etc. In this way, if some extraneous channel 5 signal is transmitted by the coarse filter into the output corresponding to channels 1 to 4 the filter of common design will not pass that extraneous signal into any of the channel outputs for channels 1 to 4. Similarly, any channel 4 signal appears in the output for the second broad frequency range corresponding to channels 5 to 8 will likewise be rejected.
According to a third aspect of the invention there is provided an optical system, having an optical processor for processing an optical signal divided into a plurality of predetermined groups of channels output on respective optical outputs, and a plurality of demultiplexers of like design connected to the optical outputs, each demultiplexer having a plurality of input guides and a plurality of output guides, each input guide having a predetermined frequency range for which optical signals input into the input guide are divided between the plurality of output guides according to frequency; wherein the optical outputs are connected to respective demultiplexers, each optical output connected to the respective demultiplexer through an input guide having a predetermined frequency range corresponding to the frequency of the respective group of channels of the optical output.
The optical processor may be a coarse filter. Thus, the invention provides, in another aspect, an optical system comprising: a coarse optical filter for dividing an optical signal into a plurality of predetermined groups of channels and outputting the groups of channels on respective optical outputs; a plurality of demultiplexers of like design, each demultiplexer having a plurality of input guides and a plurality of output guides, each input guide having a predetermined frequency range for which optical in put into the input guide are divided between the plurality of output guides according to frequency; wherein the optical outputs are connected to respective demultiplexers, each optical output connected to the respective demultiplexer through an input guide having a predetermined frequency range corresponding to the frequency of the respective group of channels of the optical output.
In another aspect there is provided an optical demultiplexer comprising: a coarse filter having a plurality of outputs for dividing an input optical signal into a plurality of predetermine broad frequency ranges of predetermined width and outputting each predetermined broad frequency range on a respective output; a plurality of arrayed waveguide gratings connected to respective outputs of the coarse filter for demultiplexing optical signals in the respective broad frequency ranges into a plural of narrow frequency ranges; wherein the arrayed waveguide gratings have properties that repeat in frequency with a period of the free special range; the arrayed waveguide gratings have first and second input waveguides, signals put into the different input waveguide rejecting and demultiplexing alternating frequency ranges of the said predetermined width, the first input waveguide rejecting the frequency ranges demultiplexed by the second input waveguide and the second input waveguide rejecting the frequency ranges demultiplexed by the first input waveguide; and the input waveguides of the arrayed waveguide gratings connected to the outputs of the coarse filter alternate with increasing frequency of the respective broad frequency range so that each arrayed waveguide grating demultiplexes the signal corresponding to the broad frequency range of the output of the coarse filter to which it is attached.
The narrow frequency ranges may be equal in size and the broad frequency ranges may each be the same integral multiple of the narrow frequency range. The narrow frequency range may correspond to a signal channel. In this way, the demultiplexer according to the invention may divide an input optical signal including a number of channels into the individual channels.
By using two-input multiple output AWGs, a greatly reduced number of switches can be provided compared to an arrangement in which switches are provided on the outputs of the AWG. Furthermore, by providing two inputs it is only necessary that the outputs cover a range of half of the FSR. This greatly reduces unevenness in power transmission through the AWG, since near to the edges of the band an AWG exhibits significantly increased loss. Further, this arrangement is colorless, i.e. Each AWG can in fact be identical and cope with the whole of the FSR, by correctly adjusting the switch.
As an alternative way of implementing this approach each AWG can be configured by connecting only the desired input of the AWG to the corresponding output. This eliminates any insertion loss caused by the switch, by making the selection of frequency range at the time of manufacture.
The coarse filter may be implemented using a splitter and a pair of filters, for example dielectric filters.
Any filter may be used as the coarse demultiplexer. For example, a polarising filter may be employed.
By providing a two stage demultiplexer optical processing can be carried out on the group of channels output by the coarse filters. The optical processing may be dispersion compensation.
The invention also relates to a node for an optical telecommunications system including a demultiplexer as set output above.
The invention also relates to an optical telecommunications system including: a transmission node providing a WDM optical signal, a receiving node for receiving the WDM optical signal; and an optical fiber connecting the transmission and receiving nodes, wherein the receiving node includes an optical demultiplexer as set out above.
In another aspect, the invention relates to a method of demultiplexing an optical signal, including: dividing the optical signal into a number of broad frequency bands; supplying each of the broad frequency bands to an AWG to split each of the broad frequency bands into a plurality of narrow frequency bands; wherein the AWGs are of common design and have one of at least two configuration, and the configuration of the AWG connected to each of the plurality of outputs of the coarse filter in ascending frequency order is cyclical.
In a yet further aspect, the invention relates to a method of manufacturing an optical demultiplexer, including: connecting an AWG to each output of a coarse filter for dividing an optical signal into a number of broad frequency bands, wherein the AWGs have a number of different configurations; and configuring the AWG such that the configuration of the AWG connected to each of the plurality of outputs of the coarse filter in ascending frequency order is cyclical.